The Excited State Properties of Thermally Activated Delayed Fluorescence Emitters: A Computational Study Towards Molecular Design

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Lighting and displays form essential parts of our daily lives and consume approximately 20% of the electricity used worldwide. Consequently, significant energy and cost savings can be achieved by improving the efficiency of these devices. Due to their lightweight, flexibility and high-performance optical and electrical properties, Organic Light-Emitting Diodes (OLEDs) are a central focus of this research and have huge potential for application in technologies such as smart phones, televisions and lighting. OLEDs are, like classic LEDs, able to transform electrical energy into visible, ultra-violet (UV) or near Infra-red (NIR) light. However, unlike LEDs, OLEDs consist of several very thin, stacked layers organic materials and do not rely on small, point-shaped single crystals. In addition, organic systems are highly attractive for mass production stemming from their ability to be deposited on a variety of low-cost substrates such as glass, plastic or metal foils, and due to their relative ease of processing. Indeed, because production costs of these devices are typically dominated by fabrication and packaging, the relatively weak van der Waals bonded organic films also create the opportunity for a new suite of innovative fabrication methods, including direct printing through the use of contact with stamps, or alternatively via ink-jets and other solution-based methods. Even though OLEDs have huge potential to achieve a higher energy efficiency than LEDs and may also be processed under more sustainable conditions, today's state of the art white OLEDs still have higher power consumption than white LEDs. In terms of efficiency, initial attempts to implement OLEDs based upon purely organic materials were restricted by the type of excited state which emits the light. Indeed, upon electrical excitation 25% of the emitting molecules are in a so called singlet excited state, while 75% are in triplet excited states. However, conventional organic materials cannot emit from the triplet excited states, meaning that only a maximum efficiency of 25% could be achieved. An extensive research effort successfully led to 2nd generation (so called phosphorescence) OLEDs that use heavy metals to promote light emission from the triplet states and, in principal, achieve 100% efficiency. However, until now the only phosphorescent materials found practically useful are iridium and platinum complexes that are unappealing for commercial applications due to their high cost and low abundance.This research proposal seeks to investigate, using multi-scale modelling, the fundamental properties crucial to molecules and materials for a new class of OLEDs that exploits thermally activated delayed fluorescence. This exploits a small energy gap between the two emitting states (singlet and triplet) so that thermal energy can transfer population from the triplet state to the singlet state. Importantly this mechanism opens the possibility to achieve, in principal, 100% efficiency and crucially precipitates the potential to return to materials containing only lighter more abundant elements, such as organic molecules. By combing quantum chemistry, molecular and quantum dynamics, this multidisciplinary approach will produce a detailed physical and chemical understanding of the material properties on a wide variety of time and length scales. Critically, these simulations will underpin our understanding of the properties that lead to their efficiency. This bottom up approach will consequently provide important insight into achieving systematic material design with the potential for vastly improved and cheaper devices.